1. Field of the Invention
The present invention relates to a liquid metal ion source used for a focused ion beam system such as a mask repair, an ion microscope, a TEM specimen preparation apparatus or an ion beam etching device for analyzing failure in a semiconductor, and a method for producing the same. In particular, it relates to a gallium ion source used suitably for an ion microscope, a focused ion beam system, a TEM specimen preparation apparatus and so on.
A focused ion beam device equipped with a liquid metal ion source used for circuit formation, mask-repairing or surface analyzing in manufacturing a semiconductor integrated circuit has been developed and spread. For the liquid metal ion source, there have been known to use various kinds of metal and alloy as ion species. Among these, a gallium ion source using gallium as ion species has been noted as being preferable for the above-mentioned mask-repairing, surface analysis and TEM specimen preparation.
2. Discussion of Background
As shown in FIG. 1, a liquid metal ion source has such a construction that a needle electrode 1 having a sharp edge at its tip and a reservoir 2 are welded to a hair-pin type heater 4 for heating the needle electrode 1 and the reservoir 2 by Joule heating, and the heater 4 is attached to supporting terminals 5 fixed to an insulating ceramic base 7. The liquid metal ion source is heated by supplying a current to the heater 4 according to requirement. As another construction of the liquid metal ion source, a needle electrode 1 is passed through a reservoir 2 as shown in FIG. 2 wherein the ion source is heated by means of heaters 4A, 4B according to requirement. In either construction, a liquid metal 3 is filled in the reservoir 2. The liquid metal 3 spreads on a surface of the needle electrode 1 and the tip of the needle electrode 1 gets wet with the liquid metal 3 in a molten state.
For the gallium ion source, a tungsten wire having a diameter of about 0.1-0.2 mm is usually used for the needle electrode and a cone portion is formed at the tip of the needle electrode by electropolishing or mechanical processing, and at the same time, the tip of the cone portion is processed to have a radius of coverture of about several xcexcm.
In such liquid metal ion source, when a positive potential is applied to the needle electrode 1 against an extractor arranged opposing to the liquid metal ion source, the liquid metal at the tip of the needle electrode forms a cone-like projection called Taylor cone when a potential difference becomes equal to a threshold value or larger, and liquid metal ions are emitted from the tip. In this case, the liquid metal ion source using an alloy as ion species is generally used at a high temperature of about 500-600xc2x0 C., for the purpose of obtaining stable ion emission for a long period of time.
However, it is preferable that the liquid metal ion source, in various ways of usage as described above, is operated at a low temperature from the viewpoint that the energy spread of emitted ions should be small so as to make the beam diameter small.
For the gallium ion source, the operation at room temperature has widely been employed since it is easy to maintain a liquid state at room temperature because of gallium having the melting point of 29.8xc2x0 C. and keeping easily a super cooling state.
However, when the operation of gallium ion source is continued at room temperature, it becomes impossible to carry out stable ion emission since the surface of gallium is contaminated by residual gases in vacuum and sputtered metals from electrodes, so that the feed of gallium from the reservoir to the tip of needle electrode is disturbed. As a result, there are problems that the ion beam emission becomes unstable and an extractor voltage becomes extremely high in an attempt to keep an amount of the ion beam emission current to a predetermined value.
With respect to the above-mentioned problems, there has been known a technique that the gallium ion source, in which the surface of gallium is once contaminated and the ion beam emission becomes unstable, can recover the original state of ion beam emission, by carrying out an operation called flashing that the temperature of the needle electrode and the reservoir is temporarily raised by means of supplying electrical current into the heater. Accordingly, a stable ion emission time between a flashing operation and the next flashing operation, i.e., a flashing interval is an index of performance of room temperature operation, and an important index on the performance of the gallium ion source.
In order to maintain stable operations, i.e., to improve availability of an ion-beam equipment such as a focused ion beam system or the like, it is necessary to stabilize ion beam emission in the liquid metal ion source for a long time. For such purpose, it is necessary to feed smoothly a liquid metal in an ion source from a reservoir through a side surface of a needle electrode to the tip of the needle electrode. Therefore, the liquid metal ion source having a long flashing interval is desired strongly.
Generally, it has been known for a needle electrode used for a gallium ion source to use a tungsten wire having a diameter of about 0.15 mm and to form a sharp edge at its free end portion by usually an electrolytic process using an aqueous solution of sodium hydroxide as electrolyte, followed by conducting mirror polishing to a side surface of tungsten wire. However, in the conventionally known gallium ion source, there was a problem that the flashing interval was extremely short as several ten hr. In such electrolytic process, there is a technique generally used that the needle electrode is dipped in electrolyte composed of a 1N sodium hydroxide (NaOH) aqueous solution, and a surface of cylindrical portion of a needle electrode, hereinafter referred to as a side surface of the needle electrode, is polished little by little to form a mirror surface while the needle electrode is moved vertically and a d.c. voltage of about 6V is applied to it.
Further, a liquid metal ion source having fine grooves in a side surface of a needle electrode to permit smooth feed of a liquid metal on the side surface of the needle electrode is disclosed in J. Vac Sci. Technol., 16(6), 1871-1874 (1979). In this liquid metal ion source, smooth feed of the liquid metal at the side surface of the needle electrode is obtainable by forming the fine grooves on the side surface of the needle electrode.
However, in such liquid metal ion source, when the operations became unstable, it was revealed that wettability of gallium on a surface of a cone portion of a needle electrode, hereinbelow referred to as a cone surface of a needle electrode, was low. Namely, there was a problem that in considering the feed of the liquid metal from the reservoir through the side surface of the needle electrode to the tip of the needle electrode, a smooth feed of the liquid metal was disturbed on the cone surface of the needle electrode, with the result that a stable ion beam emission time is short and a long flashing interval can not be obtained to a sufficiently practical extent.
The inventors of this application have made extensive studies in consideration of the above-mentioned circumstances, and have achieved the present invention by obtaining the knowledge that when the properties of a side surface of a needle electrode or a cone surface at the free end of the needle electrode are controlled so that gallium covers the needle electrode in a specified state, a flashing interval of the gallium ion source can be prolonged and stable ion beams can be obtained for a long time.
It is an object of the present invention to provide a liquid metal ion source providing a long ion beam emission time, i.e., a long flashing interval, and a stable ion beam emission property, whereby availability of an equipment using the liquid metal ion source can be improved, and productivity of a semiconductor manufacturing apparatus can be improved.
In accordance with the present invention, there is provided a liquid metal ion source which comprises a reservoir storing a metal to be ionized and a needle electrode to which the metal is fed from the reservoir wherein small pits are formed in a cone surface of the needle electrode.
In the present invention, the size of the small pits is 0.1 xcexcm-1.5 xcexcm.
In the present invention, the small pits exist in the cone surface of the needle electrode in a density of 5xc3x97104 number/mm2-5xc3x97106 number/mm2.
Further, in accordance with the present invention, there is provided a liquid metal ion source which comprises a reservoir storing gallium as a metal to be ionized and a needle electrode made of tungsten wherein the ratio of the intensities of standardized X-ray of tungsten and gallium detected on a side surface of the needle electrode is 2.0 or lower.
The liquid metal ion source mentioned above may have a heater for Joule heating at least one of the reservoir and the needle electrode made of tungsten.
Further, in accordance with the present invention, there is provided a method for producing a liquid metal ion source wherein gallium is used as a metal to be ionized and at least a side surface of a needle electrode is etched so that the ratio of the intensities of standardized X-ray of tungsten and gallium detected on the side surface of the needle electrode made of tungsten is 2.0 or lower.
In the above-mentioned method, etching is conducted by using an aqueous alkaline solution of potassium ferricyanide.
In the above-mentioned invention, gallium is used as a metal to be ionized; at least a side surface of the needle electrode is etched so that the ratio of the intensities of standardized X-ray of tungsten and gallium detected on the side surface of the needle electrode made of tungsten is 2.0 or lower, and a cone surface of the needle electrode is further etched chemically.
Further, in the above-mentioned invention, in etching the cone surface of the needle electrode, an aqueous solution of oxidative acids is used.